The present invention is directed to a method of treating a selectively galvanized steel strip, such as one-side electrogalvanized sheet, to improve the appearance and paintability of the non-galvanized areas of said strip.
Steel has been known and used for years as a construction product. However, an accepted condition of such use, depending on the environment to which the steel is exposed, was that the steel was subject to corrosive attack. In the desire to minimize such attack, workers in the art sought out methods and protective coatings for the steel. Today, zinc is one of the most widely used metallic coatings applied to steel surfaces to protect them against corrosive attack. Two principal methods of applying such coatings are (1) hot-dipping, and (2) electroplating. Hot-dipping has the advantage of cost of application. However, hot-dipping typically results in a thick coating with a rough surface, and an intermetallic alloy interface between the steel substrate and coating overlay. As a consequence, the formability and appearance of hot-dip products is limited, thus making such product unacceptable for many applications.
In contrast, electroplated zinc (1) produces smooth, thinner coatings, (2) is applied at lower temperatures, which means the base steel is less affected by such temperatures, and (3) results in little or no formation of an intermetallic alloy interface. Thus, where forming is a critical step in the fabrication of a product, electroplated zinc is the preferred product. As a consequence, electroplated zinc, or electrogalvanized steel, has become widely accepted in the automotive industry.
More particularly, one-side electrogalvanized steel sheet is a desirable product sought by the automotive industry to protect certain body panels from inside-out perforation corrosion. Although it is easier to produce one-side coated sheet by electroplating rather than hot dipping, the process is not without its problems. As steel strip travels through the plating section, the uncoated side can be pickled by the electrolyte and it can become so highly active that it forms a light oxide stain in the final rinsing and drying sections of the line. The stain affects product appearance, and phosphatability, the latter property relating to the paintability of the product. Several post-treatment methods have been reportedly used to either prevent the formation of oxide stain or to remove it after plating. One method involves protecting the uncoated side with a flash coating of zinc near the start of the plating section and then removing it anodically. Other methods involve treating the uncoated side with acids and/or brushing. However, there are several drawbacks to these methods. The flash coating method requires electroplating equipment, consumes expensive electrical power and it may leave some small amount of coating on the surface which would adversely affect paintability. The chemical cleaning of the uncoated surface with acids requires complex hardware in order to prevent attack of the zinc coated surface by these acids. Finally, brushing may result in mechanical alteration of the surface and subsequent inferior paint appearance.
The present invention, as will become apparent from the specifications which follow, avoids the costly practices associated with such prior art post-treatment methods, without deleteriously affecting the resulting product.